As pointed out in U.S. Pat. No. 4,047,368 to J. F. Peterson, the clipping receptacle of a grass catcher for a riding mower preferably comprises two or more bags mounted side-by-side on the rear of the riding vehicle, each small enough to be easily handled by one person when it is filled, but together providing a substantially large capacity that minimizes the frequency of unloading stops.
The grass catcher structure should provide for quick and easy removal of filled bags and remounting of emptied ones. However, removal of the bags should not involve detachment of small parts that can get lost in deep grass or clippings.
Clippings are guided from the mower discharge outlet towards the clipping receptacle means in a duct which is usually disposed at the side of the riding vehicle to which the mower discharges and which must necessarily have a rearward and upward inclination. Clippings therefore tend to decelerate as they move upward along that duct, so that even in the best designed grass catcher apparatus an occasional plugging of that duct can occur. Fast and easy disassembly of the duct to facilitate its unplugging is thus an important consideration. Of course it is also important for the duct to have stable support, especially at its raised rear end, and such support should be capable of sustaining the constant bumping and vibration to which the apparatus is subjected in use.
It is obviously important for the duct to be so configured that clippings move smoothly and steadily along it, without any tendency to impact against or settle on any part of the duct wall and thus form a nucleus for a growing clump that can soon plug the duct. The duct configuration must therefore take account of the trajectory of clippings discharged from the mower outlet and the dynamics of the airstream that is generated by the mower blade flights and flows along the duct, entraining the clippings and supplementing their own momentum to carry them up through the duct. It has been found that flow of clippings along the duct is not only strongly influenced by the configuration of the inlet portion of the duct but also by that of the mower discharge outlet, which functions, in effect, as the entry to the duct.
At the outlet end of the duct, clippings must be guided into the receptacle means in such a manner that they do not pile up in one part of it while the rest of it remains unfilled. Since clippings have a high angle of repose, there must be provision for a positive distribution of clippings in order to avoid underutilization of the capacity of a large receptacle means.
Prior clipping collectors reflect little or no appreciation of the fact that control of airflow in the zone downstream from the duct outlet not only influences the distribution of clippings in the receptacle means but also has a marked influence upon the flow of clippings along the duct itself. If the airstream is abruptly decelerated upon emergence from the duct outlet, its kinetic energy is converted to a static pressure head against which the flow of air along the duct must be pumped. Thus, if the duct opens abruptly into a wide chamber at its upper end, or if the airstream issuing from the duct is abruptly deflected, airflow along the duct will be correspondingly retarded and the chances for plugging of the duct will be correspondingly increased. Considered from another standpoint, the necessarily high airflow velocity along a duct having an unfavorable outlet geometry can be obtained by pumping more air into the inlet of the duct; but doing so requires more power, so that the mower must have a larger engine with a correspondingly higher fuel consumption. In this respect, it is to be observed that substantially more horsepower is consumed by a rotary mower blade in generating an air draft than in actually cutting through crop.
For guiding clippings away from the duct outlet and distributing them into the receptacle means there is usually a cover or hood that overlies the receptable means and has a connection with the rear end portion of the duct. When in place, this cover tends to interfere with easy removal of the clipping receptacle means; but if the cover is to be displaceable from its operative position, then the security of the operative connection between it and the duct must be taken into account.
The blown air that emerges from the duct must ultimately be discharged from the cover or hood through a screen or the like that prevents clippings from escaping along with the discharged air. Since grass clippings tend to be sticky, especially when they are wet, they tend to adhere to a screen if they can come into contact with it, blocking the outflow of the blown air. Therefore clippings should be guided from the duct to the receptacle means along a path that keeps them away from the screen.
Prior riding mower grass catcher devices have been deficient in that they have failed to satisfy all of the requirements and desiderata set forth above.
U.S. Pat. No. 3,716,977, to Jackson, discloses a grass catcher wherein a single large bag had a subframe around its upper edge whereby it was suspended, and the subframe was slideable horizontally to and from an operative position on a main frame, while the cover remained in its operative position. Such horizontal sliding of the loaded bag on the main frame was awkward at best. A further disadvantage of the arrangement was that removal of the cover for unplugging of the duct involved the detachment of small, easily lost caps that secured the cover in place.
U.S. Pat. No. 4,106,272, to Peterson et al, discloses a rear mounted grass catcher wherein the discharge end portion of the rearwardly and upwardly inclined clipping duct projected into the collecting bag through a hole in its front wall. The hole in the bag was obviously a cause of problems when the bag was removed for emptying, especially if the bag was filled nearly to its top, and it also complicated re-installation of the bag after it had been emptied.
U.S. Pat. No. 3,974,631, to K. H. Rhodes, discloses a grass catcher cover having side and rear walls that were apertured to serve as screens through which blown air could pass, but the cover was also formed in such a manner that clippings tended to contact the apertured walls, and especially the rear wall and one side wall, with the possibility that outflow of air would be severely restricted if the inside surfaces of those walls were not cleaned from time to time.
In the apparatus of the above mentioned U.S. Pat. No. 4,047,368, the clipping receptable bags were held in place by spring clips, and installation and removal of the bags, although not unduly difficult or troublesome, nevertheless left something to be desired in the way of convenience.
In addition to the mechanical inconveniences and disadvantages of prior grass catchers for rotary riding mowers, they were generally unsatisfactory with respect to features that affected the dynamics of clipping flow. In the grass catcher of the above mentioned Rhodes patent, for example, owing to the large area of the apertured walls in the cover, the airstream issuing from the duct practically emerged into the free air and was thus abruptly decelerated, with the results explained above. The notion that the duct should open as nearly as possible to free air is also reflected in the above mentioned Peterson et al patent, wherein there appears a statement that "In order to prevent excessive back pressure in the grass catcher (which would interfere with the movement of the grass and the air through the conduit), the grass catcher is adequately ventilated thereby permitting air to escape freely therefrom." If relatively frequent plugging of the duct was not encountered with such prior grass catchers, it was avoided by employing an engine of substantially higher power than is now known to be necessary. Where power was not expended in driving mower blades with high capacity air lift portions, it was employed to drive a vacuum type pump communicated with the outlet portion of the duct, as disclosed in U.S. Pat. No. 4,095,398 to Aumann et al.